|dc.description.abstract||Northwest Vietnam contains numerous geological structures and exposes various sedimentary and igneous rocks from Precambrian to Triassic. Most exposed rocks here have obviously undergone regional metamorphism. The area has undergone two main events of continental collision: the Indosinian event and the India-Asia collision that happened during the Permian-Triassic and the Early Cenozoic, respectively. Northwest Vietnam lies on the convergent boundary between the Indochina block to the southwest and the South China block to the northeast with distinct geological characteristics. The closure of the Paleo-Tethys led to the collision between the Indochina and South China blocks, forming the Song Ma suture zone in the Triassic. Since the Early Cenozoic, Indian and Asian continents started to collide, and this collision caused several strike-slip faults.
Most existing works on structures and tectonics are focused on either meta-ophiolitic rocks along the Song Ma Suture Zone or the Red River Shear Zone, while metasedimentary rocks in between have not been thoroughly investigated. We therefore applied a geothermometer, Raman spectroscopy of carbonaceous material (RSCM) to estimate the peak temperature in rock history, which was integrated with field and microstructural observations as well as stratigraphy information, to reconstruct the thermal evolution of Northwest Vietnam. A total of 33 samples were collected across a section from Dien Bien province to Yen Bai province and neighboring locations. The temperature results from RSCM are in the range of 215 oC to 561 oC from Neoproterozoic to Triassic formations, with the highest RSCM temperature being found in the Neoproterozoic Nam Co formation of the Song Ma suture zone. The data also show a significant difference in thermal history between Indochina and South China blocks across the Song Ma Fault indicating that the Indosinian event is more likely to dominate the peak temperature state in the area. The Triassic sediments were the last deposits on the South China continental margin and the Paleo-Tethys with minimal diagenesis before the onset of the Indonesian orogeny. The Upper-Triassic formations were metamorphosed up to around 200 oC, implying that it was underthusted to ~7 km depth before exhumation (assuming a geothermal gradient of 30 oC /km). For the rest of the Neoproterozoic to Triassic succession, we found that the apparent thermal gradient calculated from RSCM thermometer is roughly 20oC/km through stratigraphy and RSCM temperature correlation, indicating that the apparent geothermal gradient has been reduced by about one-third of its original value in the whole study area by folding prior to peak metamorphism. The area is postulated that the currently exposed continental margin sediment sequence, which is roughly 16 km thick, was considerably folded while being underthrusted to ~7 km depth where it experienced peak metamorphism, while the vertical distance between the Triassic and the Neoproterozoic formations to produce the low apparent geothermal gradient, 20 oC/km was reduced to ~11 km among the anticline and syncline cores. The iso-grade map showing the metamorphic distribution and the corresponding major structures is proposed. Northwest Vietnam will be divided into two main zones along the Song Ma Fault, and three sub-zones with the Na Hieng Fault, and Song Da Fault as major structures along with the local metamorphic Ta Khoa zone.||en_US|